The present invention relates to electrical cable insulation. More particularly it relates to a new and improved shielding tape for use in insulating underground cables which provides an electrostatic shield and moisture barrier to the cables.
Polyethylene is widely used as high-frequency insulation on power and coaxial cables and as primary insulation on telephone cables, multiconductor control cables, television lead-in wire and appliance wire. The characteristics of polyethylene include: good electrical properties; chemical resistance to solvents, acids and alkalies; toughness and flexibility even at low temperatures; good barrier properties; adaptability to many processing techniques; and relatively low cost.
A problem arises with polyethylene insulated conductors and cables where long-term underground installation is required. When cables such as telephone and high voltage power cables are installed under ground by being buried directly in the soil, the outer polyethylene jackets of such cables may be subjected to damage from the rigors of installation, rocks, rodents, lightning, frost and water. Prolonged exposure to the corrosive effects of subterranean percolating water has proved to be a major problem. The manufacture and installation of underground cables is so expensive that insulations which can withstand environmental assault for more than thirty years are particularly desirable. Polyethylene jackets alone have not proved satisfactory to the task.
More recently, it has become known that improved underground cables may be provided by assembling insulated conductors in a core and surrounding it first with a shield and then a polyolefin jacket. The term "shield or shielding tape" as used herein means a relatively thin layer of any metal, bare or coated, which can provide mechanical protection and electrostatic and electromagnetic screening for the conductors in the core of electrical power and communications cables. The term is also referred to in the art as metallic strip, metallic foil, metallic screen, metallic barrier, metallic sheath, shielding material, etc., which terms are often used interchangeably.
In order to improve the corrosion resistance of the shielding tape of bare metal, a special adhesive polyethylene film may be applied to cover one or both sides of the metallic strip. Such an adhesive polyethylene film is disclosed in U.S. Pat. No. 3,233,036, wherein a graft copolymer of polyethylene and a monomer with a reactive carboxyl group, such as acrylic acid or acrylic acid ester, is applied to both sides of an aluminum foil tape. These laminated tapes are preferable to uncoated metallic tapes or to plain polyethylene coated tapes because, although polyethylene films generally satisfy the requirements of electrical resistivity and resistance to chemicals and moisture, they develop only a mechanical bond with the metallic tapes based on friction-type adhesion, and delamination of polymer film from the metal tape does occur. The carboxyl components of the copolymer, on the other hand, have the property of forming chemical bonds with the metal to provide improved bonding of film to metal.
In U.S. Pat. No. 3,586,756, a more economically attractive alternative shielding tape is disclosed having a protective film layer in the form of a low cost homopolymer with a concentration of reactive groups (carboxyl) in the part of the layer adjacent the metallic foil and only in that part of the protecting layer. Other shielding tapes having similarly improved resistance to delamination are described in U.S. Pat. No. 4,049,904 and U.S. Pat. No. 4,092,452.
More recently, the demand for higher voltage cables has increased. In power cable applications for transmitting relatively high voltage loads such as 5 kV and above, ionization resistance, sometimes referred to as corona resistance, may become a problem because it may lead to the premature breakdown of the cable insulation. Accordingly, there is currently a need for insulation having superior properties over extended exposure to high voltage stess.
It has presently been discovered that improved polyolefin based cable shielding tapes incorporating a voltage gradient (an electrostatic shield) and a moisture barrier, and having resistance to delamination may be prepared if the polyolefin is rendered semiconductive and the metallic foil is pre-treated with certain adhesion promoting materials.